Conventional reflection seismology involves generating acoustic waves from a seismic energy source and then detecting reflections of those waves from interfaces in the earth is formation being analysed. When a velocity interface is encountered, the seismic waves are partially reflected back to the surface, where they are detected and recorded. The time taken for the reflected energy to return travelling at the speed of sound through the earth (as attenuated by the different types of rocks and formations encountered) indicates the depth of the reflecting structure. Geophones are used to measure the vertical and/or horizontal components of the reflected seismic waves.
U.S. Pat. No. 4,904,942 discloses a different scheme for seismic analysis referred to as “electroseismic” prospecting. In this approach, seismic energy is converted into electromagnetic energy. This conversion is believed to take place as a result of distortion or breakage of bonds between a fluid and the surface of a porous structure containing the fluid. Rapid movement of the fluid caused by an incident acoustical wave front disturbs these dipoles and induces an electromagnetic response. In contrast to a reflected acoustic wave, the electromagnetic wave generated by the electromagnetic response travels at the speed of light through the earth (as attenuated by the materials encountered) with respect to the rock formations.
EP 1577683 describes a method for characterising a formation including exciting the formation with an acoustic wave and measuring a seismo-electromagnetic response, and then exciting the formation with an electromagnetic field and measuring an electromagnetic-seismic response.